Wind turbines are resilient machines designed to last decades. However, they are not immune to the dangers like lighting strikes, extreme weather, and fire. The wind turbines often get stuck by the lighting. They are tall structures with heights ranging from 25 meters to 100 meters and even more. With their size and construction, they are desirable targets for lightning. Even though they are designed to resist plenty of lightning strikes, but repeated long term exposure can certainly cause significant damage
How often do wind turbines get hit by lightning?
On average, wind turbines are hit by lightning about 10 times in a year. This is mainly due to their height and remote location. Tall Commercial turbines are more susceptible to lighting compared to the shorter residential ones. However, not all lightning strikes are detrimental for turbines.
This number can significantly vary based on the height and location of the turbines. In places like Japan and Sweden, with higher instances of lightning in the winter. Turbines can face more lightning strikes annually. An extreme example is of a wind power plant situated on the sea of Japan, where a turbine was hit 100 times in a single year by lightning strikes. (Source)
Why lightning strikes the turbine?
As the water evaporates, it goes up in the atmosphere and forms clouds. In these clouds, the low temperature turns water into Ice. These ice particles move up and down, colliding with each other generating the charge. This charge eventually grows to several billion volts of electricity.
At this point, a lightning flash can build up between two clouds or between a cloud and a structure on the ground.
There are two main types of flashes.
- Negative Flash – When the charge propagates from the negative cloud to the positive ground, it is called a negative flash. This is the most common flash type and represents 90% of the flashes.
- Positive Flash – When the charge moves from a positive cloud to negative ground, we call it a positive flash. 10% of the flashes are credited to this kind. Positive flashes, in general, carry a significantly higher charge compared to negative flashes.
The flashes travel as 20-100m quasi-random jumps. As the flash approaches the ground, it seeks the shortest path to do so. Even though it is not necessary that it will choose the tallest object, but in most cases, it does.
Tall structures like skyscrapers, trees, and wind turbines are very attractive for the lightning as it provides it with a much-needed shortcut to the ground. (Source)
What happens when a wind turbine gets struck by lightning?
It might look simple, but a wind turbine houses a lot of technology in its structure. It has
- A transformer that converts the generated voltage to the grid voltage.
- A frequency converter that changes the electrical frequency to 50 or 60 Hz depending on where you live.
- A switchgear mechanism.
- Pitch control systems and mechanisms to control the blade angles.
All these systems need to function in tandem for everything to go well. Damage to any of these can haul the operations, and in the case of substantial multimillion-dollar turbines, it can cause significant financial loss.
Most modern turbines come with lightning protection systems. Though earlier, that was not the case. In older versions, though, there was limited protection that made them vulnerable to damage. Most lightning strikes will not harm present-day turbines, but if they do, they can create many problems like
- Damage By Excess Current – Whenever lighting currents pass through a conductor, often there is a heating effect accompanied by magnetic forces. The amount of heat generated varies by materials, and if it is too much, it can also start a fire. Copper and aluminum can carry higher amounts of energy without enormous heating. Steel, on the other hand, can heat up quickly.
- Magnetic – Electricity and magnetism go hand in hand—any two conductors parallel to each other produce a magnetic field. If the current is flowing in the same direction in these conductors, they will attract each other. If it is following in the opposite direction, they will repel each other.
How does that relate to turbine and lightning? Well, there are conductors in the turbine which are thin and are bent. If the bent is less than 90 degrees, it creates a repulsive force. When lightning passes through these, it generates impulsive forces. These forces can bend, buckle or even break these sections.
- Explosive effects – The inner parts of the blades can develop electric arcs when lightning strikes. These arcs form mainly due to a hole or a flaw in the blade. These arcs are enclosed and create a considerable overpressure that can have explosive effects that can easily damage or destroy the blade.
How can we protect wind turbines from lightning?
The first step in protecting a wind turbine against lightning strikes is to assess the area’s local lightning occurrence. You can get this information from the authorities like national weather bureaus and other people who work with turbines. Years of experience and research have taught engineers that turbines can be effectively protected against lightning strikes. The leading research is compiled in a report called IEC 61400-24.
We have to understand that we are protecting the structure from two primary things
- High current or arc damage at the point of attachment
- Damage from current flowing throughout the structure
There are three methods to achieve it.
- Give lightning another target to strike, so it spares the vulnerable section. This is usually an additional larger structure that can be used to protect the nacelle if blades are secure. However, it cannot be used for blades as the required structure would be massive.
- Safeguard the parts which are most likely stuck by the lightning using protective attachments
- Give the large current induced by lightning an additional path so it can safely move to the ground. This applies to parts made of high resistance materials like reinforced carbon. By providing an alternate route to the current, You can avoid most damage.
Let’s talk about specific parts of the turbine and what measures can be taken to secure them.
Blades
Blades are generally made up of fiberglass and wood. They don’t offer a conductive path for the current. Through numerous observations and experience, we have learned that blades are often struck by lightning, and unprotected ones can suffer catastrophic destruction. As we discussed before, blades are also vulnerable to explosive arcs. Researchers believe that the presence of dust and moisture increases the risk of arc formation. The explosive vaporization of water can also cause dramatic pressure changes.
Even though blades are made of carbon, it is equally tempting for lighting as a metal; the blade material’s higher resistance is one of the main reasons for their vulnerability. However, if we add a conducting material like metal and connect it to the ground, it changes how lighting reacts to the blade.
Blades can be secured against lightning strikes by
- External metallisation
- Internal Conductors
- External conductors
It is vital to ensure that the cross-section area of the conductor is sufficient to endure the lightning. The standard methods of protection include covering the blade with a mesh, foil, or a metallic tip cap. Even with these measures, lightning will strike the blades, but it will not cause any noticeable harm that can affect the functioning and strength of the structure.
Nacelle
It is the part located right behind the blades. Nacelle houses many essential elements and sometimes working personnel; therefore, it’s crucial to protect it from lightning. Here are a few measures to secure a nacelle from a lightning strike
- For primary protection, the nacelle should be made up of a conduction material bound to the adjacent structure.
- If the blades are protected, they offer significant protection to the nacelle. To secure it even further, an additional conductor can be installed at the rear end. This conductor should be atleast 2 meters in height. In case the blades are not protected, you can add another conductor to the front end and connect both conductors using a wire.
- In a nacelle made of non-conducting material, the sensitive wirings and control circuits should be adequately shielded. Wiring should run close to the metal structure to avoid closed loops.
- To protect the working personnel, you can install metal strips on the surface of the nacelle and connect them to the base. This cheap method of protection is very effective.
Bearings
The lightning that strikes the blades enters the tower through the slow-speed shaft bearing. The lighting cannot destroy the bearing, but it can undoubtedly cause pitting damage (Localised destruction resulting in more minor indentations and holes). This can result in increased noise and reduced life of the bearing. To protect the bearings, you can
- Use pre-loaded bearings. This is the most common method. Even though there are still damages, but they are in the acceptable range
- You can also bypass the bearing and send current directly to the shaft by using sliding contacts or spark gaps.
Electronics
The electronic parts housed in the turbine-like transformers, changers, etc., can overload a lightning strike. In these conditions, they can stop working and even catch fire. You can protect these parts in two ways.
- Suppression Devices – The methods mentioned above can be further strengthened by transient suppression devices, mainly gas discharge tubes and metal oxide varistors. These should be carefully installed in the strategic locations on the structure.
- Earthing – There are specific earthing systems for tall structures like turbines. The most common ones consist of interconnecting ring earth electrodes. They reduce the dangerous step voltage around the ground and prevents surplus current in wires and instruments.